Studies will be performed in order to elucidate the mechanism by which the trace mineral selenium enhances the development of dental caries. The frequency of dental caries will be measured in rats receiving selenium in either the inorganic (Na2SeO3) or the organic form (selenomethionine) during the period of tooth formation. Physical (microhardness values, tooth dimensions, scanning electron micrographs) and chemical determinations will be made on teeth exposed to selenium during their formation. The selenium content of various fractions of human teeth will also be measured.